Implement parameter validation for input_fields

[lucassz]

Implement parameter validation for input_fields, both for TaxBrain and for the partial equilibrium model.

[hdoupe]

@lucassz This looks good. Can you add a test demonstrating the validation behavior? There aren't any specific tests for the forms. You can create a new file taxbrain/tests/test_forms.py with the new tests. A test with OK parameters and a test with unknown parameters should be sufficient.

You can look to webapp/apps/test_assets/test_models.py and webapp/apps/taxbrain/tests/test_models.py for some relevant examples.

[lucassz]

@hdoupe Done. It seemed more relevant to add it to test_views.py since there was similar code there already.

[hdoupe]

@lucassz This shouldn't be xfail-ed. That means that py.test will ignore it if the test fails.

[lucassz]

@hdoupe Thank you. I think I had understood the purpose of xfail.

[hdoupe]

@lucassz This looks great. Thanks for taking care of this and for finding a much simpler way to solve the problem.

[hdoupe]

@lucassz This should be good to go once you knock out the merge conflicts.

[lucassz]

@hdoupe Happy to do it. Should be good to go now.

[hdoupe]

Great, thanks @lucassz